Methods for removing heavy metals from aqueous extracts of tobacco

ABSTRACT

Disclosed is a method for removing one or more heavy metals from an aqueous plant extract, comprising: contacting the aqueous plant extract with, and sorbing at least a portion of the one or more heavy metals on, at least one sorbent selected from the group consisting of: one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more finely ground γ-aluminas, one or more photocatalytic titanium dioxide particles, one or more Au-anatases, ceria, and combinations thereof, to form a mixture of sorbent and heavy metal-depleted aqueous plant extract; and separating the sorbent from the mixture to provide a heavy metal-depleted aqueous plant extract.

SUMMARY

Disclosed is a method for removing one or more heavy metals from an aqueous plant extract, comprising: contacting the aqueous plant extract with, and sorbing at least a portion of the one or more heavy metals on, at least one sorbent selected from the group consisting of: one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more finely ground γ-aluminas, one or more photocatalytic titanium dioxide particles, one or more Au-anatases, ceria, and combinations thereof, to form a mixture of sorbent and heavy metal-depleted aqueous plant extract; and separating the sorbent from the mixture to provide a heavy metal-depleted aqueous plant extract.

In a particular embodiment, the method is particularly suited to processing tobacco as the plant material, and to removing one or more of the heavy metals cadmium, arsenic, lead, nickel, or selenium, which can be taken up by the tobacco plant from the soil. The resulting heavy metal-depleted tobacco extract can be used as a flavorant, or combined with other materials to form a heavy metal-depleted tobacco product. In a particular embodiment, the heavy metal-depleted tobacco extract can be combined with plant materials, particularly tobacco.

In a more particular embodiment, the heavy metal-depleted tobacco extract can be combined with some or all of the tobacco from which the extract was originally obtained, providing a heavy metal-depleted tobacco product. Such a product can be used in smoking articles, or in smokeless tobacco products.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a graph showing results for cadmium removal from an aqueous plant extract according to one embodiment of the method described herein.

DETAILED DESCRIPTION

As used herein, the terms “plant” or “plant material” denote any plant or derivative thereof, such as whole plants, parts thereof, such as stems, leaves, roots, and/or seeds, shredded, cut, ground, or otherwise processed plants, reconstituted plant material, and the like. The plant material may be in solid form, or in the form of a slurry or extract. The term includes, but is not limited to, plant material that comprises a botanical to be consumed by humans, such as tobacco or an herbal material.

As used herein, the term “aqueous plant extract” denotes a liquid or solid material that has been obtained by contacting a plant material with an “aqueous extractant” (used herein to denote an extractant comprising water, or aqueous solution or suspension, but which may also contain other liquids if necessary or desirable), for a time and under conditions sufficient to remove at least a portion of at least one component from the plant material and into the extractant. The term “extracted plant material” denotes the plant material remaining after such an aqueous extraction has occurred, and the extractant has been removed.

As used herein, the term “sorbent” denotes a solid material capable of removing one or more components from a liquid phase by “sorption” or “sorbing” the component. The terms “sorption” and “sorbing” denote the taking up of the component or components, whether by absorption, adsorption, ion exchange, or other mechanism.

As used herein, the term “heavy metal” denotes a metal having an atomic weight greater than that of sodium. It includes, but is not limited to, magnesium, aluminum, calcium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, germanium, arsenic, selenium, strontium, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium silver, cadmium, indium, antimony, tin, barium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, tellurium, lead, bismuth, and polonium. In particular, it includes divalent and trivalent ions of these metals, particularly those of copper, zinc, cadmium, mercury, arsenic, lead, nickel, and selenium.

As used herein, the term “surface activated titanium oxide” denotes a titanium oxide that has been treated to contain a number of surface hydroxyl moieties. A nonlimiting example of such an surface activated titanium oxide is Metsorb HMRG® (available from Graver Chemical). As used herein, the term “surface activated titanium oxide” does not include a photocatalytic titanium dioxides or an Au-anatase, which are described separately herein.

As used herein, the term “chitosan” denotes one or more of the family of deacylated glucosamine polysaccharides known as chitins.

As used herein, the term “calcium phosphate” includes hydroxyapatites in any form. Non-limiting examples include Macro-Prep® Ceramic Hydroxyapatite Type I and Hydroxyapatite Bio-Gel® HTP Gel.

As used herein, the term “mercaptoalkyl-substituted silica gel” denotes silica gels that have been modified to contain covalently bonded mercaptoalkyl surface groups by reaction of the silicate with a mercaptoalkyl silane. A non-limiting example of such a material is SilicaHash® mercaptopropyl modified silica gel. As used herein the term “mercaptoalkyl-substituted silica gel” does not include a mercaptoalkyl-substituted mesoporous molecular sieve which is described separately herein.

As used herein, the term “mercaptoalkyl-substituted mesoporous molecular sieve” denotes a molecular sieve material that has been modified to contain covalently bonded mercaptoalkyl surface groups. Silicate molecular sieves having substantially uniform pore sizes ranging between about 2 and about 50 nm, more particularly about 5.5 nm, and a mean surface area of about 500 m²/g. are preferred. Particularly preferred are sorbents containing mercaptopropyl moieties covalently bonded to mesoporous silicates at loadings ranging between about 1% and about 20%, more particularly between about 4% and about 16%. As used herein the term “mercaptoalkyl-substituted mesoporous molecular sieve” does not include a mercaptoalkyl-substituted silica gel, which is described separately herein.

As used herein, the term “finely ground” denotes having an average particle size of about 0.125 in or less.

As used herein, the term “γ-alumina” denotes an anhydrous aluminum oxide.

As used herein, the term “photocatalytic titanium dioxide” denotes titanium dioxide, generally in the form of anatase, that is capable of forming an electron-hole pair under UV light. A non-limiting example of such a material is Hombikat (Sachtleben). As used herein, the term “photocatalytic titanium dioxides” does not include the surface activated titanium oxide or the Au-anatase described separately herein.

As used herein, the term “Au-anatase” denotes a material prepared by precipitation of about 2 wt % Au onto titanium dioxide, such as photocatalytic titanium dioxide, such as Hombikat, using in situ precipitation with urea as the hydrolyzing agent. As used herein, the term “Au-anatase” does not include the photocatalytic titanium dioxides or surface activated titanium oxides described separately herein.

As used herein, the term “heavy metal-depleted” denotes a material that has undergone a method for removing heavy metals, and as a result has a heavy metal content lower than the heavy metal content of the material before undergoing the process.

As used herein, the term “about” when used in connection with a stated numerical value, such as an amount or range, denotes what would be understood by one skilled in the art, namely somewhat more or somewhat less than the stated value, up to a variation of ±10% of the stated value.

The embodiments described herein provide methods for removing certain targeted compounds, such as heavy metals, from aqueous extracts of plant material, so that if the aqueous extracts are later recombined with the extracted plant material, or are otherwise utilized, they are depleted of the targeted compounds.

One possible application for such a method is in the removal, from material harvested from plants, of cadmium, mercury, arsenic, selenium, and other heavy metals. These metals are typically divalent or trivalent, and can be taken up by growing plants from the soil. A particularly preferred embodiment is the application of such a method to removal of such metals from tobacco.

In such an embodiment, the tobacco or other plant material can first be extracted with an aqueous extractant (e.g., water, or an aqueous solution or suspension), by contacting the plant material with the aqueous extractant under conditions suitable for extracting at least some of the constituents of the plant material into the aqueous extractant, forming an aqueous plant extract. The resulting aqueous extract is separated from the extracted plant material, and is treated with a sorbent to remove at least some of one or more of the heavy metals that may be present in the aqueous extract. The resulting heavy metal-depleted aqueous plant extract can then advantageously be used as a flavorant, recombined with the extracted plant material and used as part of a smoking material, a smokeless product, or put to other uses.

In a particular embodiment, the plant material is tobacco or a tobacco substitute. Examples of suitable types of tobacco materials may include, but are not limited to, flue-cured tobacco, Burley tobacco, Maryland tobacco, Oriental tobacco, rare tobacco, specialty tobacco, reconstituted tobacco, agglomerated tobacco fines, blends thereof and the like. Preferably, the tobacco or tobacco substitute is pasteurized. Some or all of the tobacco material may be fermented.

Further, the tobacco or tobacco substitute may be used in any suitable form, including shreds and/or particles of tobacco lamina, processed tobacco materials, such as volume expanded or puffed tobacco, or ground tobacco, processed tobacco stems, such as cut-rolled or cut-puffed stems, reconstituted tobacco materials, blends thereof, and the like. Genetically modified tobacco may also be used.

The sorbents disclosed herein are relatively inexpensive to produce or obtain, and avoid the difficulties associated with complex and expensive synthetic polymeric sorbents.

Plant extracts tend to be complex mixtures of inorganic and organic materials, some of which are water soluble, and some of which are insoluble but present as suspensions or emulsions. In order to have an economically feasible sorbent, it is generally desirable that the sorbent be sufficiently selective toward the targeted heavy metals, so that sorbent sites are not taken up by non-targeted constituents that decrease the loading of the sorbent for targeted constituents. Accordingly, removal of heavy metals from an aqueous extract of plant material presents challenges different from those posed by simply purifying industrial waste water or other processes for removing heavy metals from simpler, more predictable aqueous solutions, and it is not generally possible to predict whether a particular sorbent that functions to remove heavy metals from a simpler aqueous solution will function effectively and efficiently to remove heavy metals from a plant extract.

Similarly, the use of sorbents to remove targeted components from the gas or vapor phase is not predictive of the ability of those sorbents to remove the same targeted components in the liquid phase or from an aqueous solution. This is partially due to the different kinetics of vapor phase and liquid phase systems, and partially due to the additional constituents present in an aqueous extract that can compete with the targeted constituents for sorbent sites.

In one embodiment there is provided a method for removing one or more heavy metals from an aqueous plant extract, comprising contacting the aqueous plant extract with at least one sorbent selected from the group consisting of one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more γ-aluminas, preferably finely ground γ-aluminas, one or more photocatalytic titanium dioxides, one or more Au-anatases, ceria, and combinations thereof.

In one preferred embodiment, the sorbent is selected from the group consisting of mercaptoalkyl-substituted mesoporous molecular sieves, photocatalytic titanium dioxide particles, Au-anatase, and finely ground γ-alumina.

In another preferred embodiment, the sorbent is selected from the group consisting of ceria and mercaptoalkyl-substituted mesoporous molecular sieves. In a particularly preferred embodiment, the sorbent is ceria. In another particularly preferred embodiment, the sorbent is photocatalytic titanium dioxide particles. In another particularly preferred embodiment, the sorbent is finely ground γ-alumina. In another particularly preferred embodiment, the sorbent is Au-anatase. In another particularly preferred embodiment, the sorbent is surface activated titanium oxide particles. In another particularly preferred embodiment, the sorbent is calcium phosphate.

A particularly preferred form of surface activated titanium oxide particles include titanium oxide/titanium hydroxide particles, such as those sold under the tradename Metsorb HMRG®. Particularly preferred forms of calcium phosphate are hydroxyapatites sold under the tradenames Macro-Prep® Ceramic Hydroxyapatite Type I or Type II and Hydroxyapatite Bio-Gel® HTP Gel. A particularly preferred form of mercaptoalkyl-substituted silica gel is a mercaptopropyl substituted silica gel, more particularly that sold under the tradename SilicaFlash®. A particularly preferred form of finely ground γ-alumina is 0.125 in diameter pellets obtained from Alfa Aesar. A particularly preferred form of titanium dioxide particles is TiO₂ Hombikat (100% anatase)- and/or TiO₂ on which gold particles have been deposited (gold Hombikat), optionally prepared by in situ precipitation with 2% Au using urea as a hydrolyzing agent. A particularly preferred form of ceria is CeO₂ obtained from Arris International.

Another embodiment provides a method for producing a metal-depleted aqueous tobacco extract, comprising:

(a) contacting tobacco plant material with an aqueous extractant to form a first mixture comprising aqueous extractant and tobacco plant material;

(b) separating an aqueous tobacco extract from the first mixture, leaving behind the extracted tobacco plant material;

(c) contacting the aqueous tobacco extract with at least one sorbent selected from the group consisting of one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more finely ground γ-aluminas, one or more photocatalytic titanium dioxide particles, one or more Au-anatases, ceria, and combinations thereof to form a mixture of sorbent and aqueous tobacco extract; and

(d) separating the sorbent from the mixture, to form a metal-depleted. aqueous tobacco extract.

Another embodiment provides method for producing a metal-depleted tobacco product, comprising:

(a) contacting tobacco plant material with an aqueous extractant to form a first mixture comprising aqueous extractant and tobacco plant material;

(b) separating an aqueous tobacco extract from the first mixture, leaving behind an extracted tobacco plant material;

(c) contacting the aqueous tobacco extract with at least one sorbent selected from the group consisting of one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more finely ground γ-aluminas, one or more photocatalytic titanium dioxide particles, one or more Au-anatases, ceria, and combinations thereof to form a mixture of sorbent and aqueous tobacco extract;

(d) separating the sorbent from the mixture, to form a heavy metal-depleted. aqueous tobacco extract; and

(e) recombining the metal-depleted aqueous tobacco extract with the extracted tobacco plant material to form a heavy metal-depleted tobacco product.

The heavy metal-depleted tobacco product obtained from this process can be used to replace some or all of the tobacco typically used in smoking articles and/or in smokeless tobacco products. When used in smoking articles, the heavy metal-depleted tobacco product can form some or all of the tobacco used in cigarettes, cigars, cigarillos, and other smokable tobacco products.

The cigarettes may be traditional cigarettes, electrically heated cigarettes, or cigarettes having a fuel element in the tobacco rod. Traditional cigarettes generally have a substantially cylindrical rod shaped structure which typically includes a roll or column of smokable material, such as shredded tobacco, surrounded by a paper wrapper. Many types of cigarettes may have a cylindrical filter portion aligned in an end-to-end relationship with the tobacco rod. The filter portion can comprise one or more plugs formed from a cellulose acetate tow circumscribed by a paper material known as “plug wrap” thereby forming a “filter plug.” Typically, the filter portion is attached to one end of the tobacco rod using a circumscribing wrapping material known as “tipping paper.”

Cigarettes for electrical smoking systems as described in commonly-assigned U.S. Pat. Nos. 6,026,820; 5,988,176; 5,915,387; 5,692,526; 5,692,525; 5,666,976; and 5,499,636. Other non-traditional cigarettes include those having a fuel element in the tobacco rod as described in U.S. Pat. No. 4,966,171.

The methods described herein can be more clearly understood by reference to the following non-limiting examples.

Example 1

Deionized water is maintained at 70° C. for one hour. 360 g of DBC bright tobacco are extracted with the deionized water and the resulting extract is divided into 30 mL aliquots. Separate aliquots are each contacted with a 1 gram sample of a different sorbent for one hour, with one aliquot serving as an untreated control. The sorbents are removed from the aliquots by centrifugation, and the treated aliquots are analyzed for the presence of cadmium and other heavy metals using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results are given in Table 1 below. The LSQ of cadmium is 1.33 μg/L.

TABLE 1 Cadmium by Sample Description ICP-MS (μg/L) % Reduction Untreated control 27.5 0 Divergan ® HM (polyvinyl imidazole, <1.33 >95 BASF) Metsorb ™ HMRG <1.33 >95 Chitosan 1.5 95 Macro-Prep Ceramic Hydroxyapatite <1.33 >95 Type I Hydroxyapatite Bio-Gel http Gel <1.33 >95

Example 2

2720 mL of deionized water is maintained at 70° C. for one hour. 360 g of DBC bright tobacco is extracted with the deionized water, producing a ratio of aqueous extract to tobacco of 7.6 mL/g. Separate 30 mL aliquots of this extract are treated with 1 g of one of the sorbents indicated below by contacting them for 1 hour. The sorbent is then removed and the treated extract analyzed for cadmium and other heavy metals by ICP-MS. The results are given below in Table 2.

TABLE 2 Cadmium by Sample Description ICP-MS (μg/L) % Reduction Untreated Control 144.00 0 Metsorb HMRG 11.10 92 Chitosan 12.00 92 Macro-Prep Ceramic Hydroxyapatite 7.29 95 Type I Si-Thiol MTP Silica Gel 13.30 91 MTP-SBA-15 (4.0% MTP Loading) 9.00 94 MTP-SBA-15 (8.0% MTP Loading) 2.20 98 MTP-SBA-15 (16.0% MTP Loading) 1.66 99

MTP-SBA-15 is a mercaptopropyl-substituted mesoporous molecular sieve having a 55 Å pore size, and MTP loadings of 4.0%, 8.0%, or 16.0%, as indicated in Table 2. It is described in more detail in U.S. Patent Application Publication 2006/0130855, published Jun. 22, 2006, the entire contents of which are incorporated herein by reference. All of the sorbents tested show at least 90% reduction in cadmium as compared to the untreated control extract. Reduction in cadmium reached 98% or higher for MTP-MMS when the MTP loading was 8.0% or 16.0%. In addition, Metsorb HMRG, chitosan, and Macro-Prep Ceramic Hydroxyapatite Type I each provided at least a 67% reduction in lead, as compared to the untreated control.

Example 3

Deionized water is maintained for one hour at 70° C., and then used to extract 360 g of DBC bright tobacco. The resulting aqueous extract is divided into 5 aliquots of equal volume. Each aliquot is treated with 40 g/L of extract with the sorbents: Metsorb HMRG, SilicaFlash, Macro-Prep Ceramic Hydroxyapatite Type I, and MTP-SBA-15 (16.0% MTP loading). After 1 hour of contacting with the extract, the sorbent is removed from the extract by centrifugation, and the treated extract is freeze-dried, as is the untreated control. The extracted tobacco material is air-dried to about 5% OV in a ventilated space and divided into 5 equal portions. Each of the freeze-dried materials is separately dissolved in twice the amount of deionized water (v/w) and the resulting solution is sprayed onto the dried extracted tobacco material to form reintegrated tobacco. The sprayed plant material is hand made into a cigarette, smoked, and the amount of cadmium in the mainstream smoke will be analyzed by ICP-MS.

Example 4

100 mL of DBC tobacco blend cut filler extract solution is prepared by dissolving 500 mg of dried tobacco extract in 100 mL of deionized water. 2 g of each sorbent set forth in Table 3 was suspended in this solution while stirring at room temperature. The samples (15 mL each) were collected at the time intervals indicated in Table 3 (i.e., after 15 minutes, 1 hour, and 24 hours of stirring), filtered with 25 mm GDX disposable filters, and analyzed using Atomic Absorption Spectroscopy/Inductively Coupled Plasma Mass Spectrometry (AAS/ICP-MS) The results are provided in Table 3. MMK denotes montmorillonite clay K10 (Aldrich Chemicals); NaY denotes sodium Y-zeolite (Aldrich Chemicals); HY denotes protonated NaY (prepared by urea hydrolysis of NaY, followed by calcination at 550° C.); 2% Au/TiO₂ denotes gold Hombikat catalyst prepared using urea as a hydrolyzing agent; and γ-Al₂O₃ denotes finely ground gamma-alumina powder in the form of 0.125 in. diameter pellets (Alfa Aesar).

TABLE 3 Metal Concentration (μg/L) 15 min stirring Sorbent 1 hr stirring 1 day stirring 2% γ- Metal Control MMK NaY HY Control MMK NaY HY Au/TiO₂ Al₂O₃ As 2.09 14.7 2.93 3.44 2.67 23.0 4.21 2.30 0.77 1.10 Cd 5.69 2.2 2.94 0.89 5.93 0.76 1.93 1.05 0.41 0.45 Pb 3.66 11.3 5.34 5.21 4.89 17.3 4.90 10.3 2.27 0.16 Ni 24.3 20.6 20.9 34.0 20.4 37.0 23.4 42.6 11.9 13.9 Se 2.09 3.30 2.48 1.92 1.99 3.10 2.11 1.60 1.71 1.88 While each of the sorbents evaluated showed sorption of at least one of the heavy metals tested, only 2% Au/TiO₂ and γ-Al₂O₃ showed sorption of each of the heavy metals tested. Moreover, 2% Au/TiO₂ showed significantly higher sorption of As, Cd, and Ni, as compared to sorption by MMK, NaY, and HY. γ-Al₂O₃ showed significantly higher sorption for each of the heavy metals tested, as compared to MMK, NaY, and HY. Finally, these increases in sorption occurred over a much shorter time frame than sorption by MMK, NaY, and HY, indicating that the rate of sorption for 2% Au/TiO₂ and γ-Al₂O₃ is higher than for the other sorbents tested. These results indicate that 2% Au/TiO₂ and γ-Al₂O₃ are good sorbents for removing heavy metals from tobacco blend extracts.

Example 5

The procedure described above in Example 4 was repeated, using the sorbents described in Table 4 below, and using a stirring time of 15 minutes.

TABLE 4 Metal Concentration (μg/L) 15 min stirring Sorbent Metal Control CeO₂ TiO₂-Hombikat γ-Al₂O₃ As 2.98 1.14 0.65 0.84 Cd 4.96 0.77 0.16 0.61 Pb 1.68 0.63 0.84 1.19 Se 2.47 2.12 1.87 1.87 The results provided in Table 4 indicate that the selectivity for TiO₂— Hombikat, based upon percent reduction, is in the order Cd>As>Pb>Se. For γ-Al₂O₃, the selectivity order is the same. For ceria, the selectivity order is Cd>Pb>As>Se.

Example 6

50 grams of tobacco is treated with 350 mL water at 70° C., followed by filtration. The resulting extract is treated with the amounts of γ-Al₂O₃ (as pellets) given in Table 5 below. Cadmium concentrations in each of the treated samples and control were measured as in Examples 4 and 5, and the results are provided below in Table 5 and graphically in FIG. 1.

TABLE 5 Cd concentration γ-Al₂O₃ (μg/L) % Reduction vs (grams) Cd¹¹¹ Cd¹¹⁴ control Control 5.678 5.702 0 10 5.098 5.101 10.5 20 5.075 5.100 10.6 30 4.198 4.215 26.1 40 3.769 3.780 33.7

The examples and specific embodiments described herein are intended to provide for better understanding, and not to limit the scope of, the methods described herein and in the appended claims. 

1. A method for removing one or more heavy metals from an aqueous plant extract, comprising: contacting the aqueous plant extract with, and sorbing at least a portion of the one or more heavy metals on, at least one sorbent selected from the group consisting of: one or more surface activated titanium oxide particles, one or more chitosans, one or more calcium phosphates, one or more mercaptoalkyl-substituted silica gels, one or more mercaptoalkyl-substituted mesoporous molecular sieves, one or more finely ground γ-aluminas, one or more photocatalytic titanium dioxide particles, one or more Au-anatases, ceria, and combinations thereof, to form a mixture of sorbent and heavy metal-depleted aqueous plant extract; and separating the sorbent from the mixture to provide a heavy metal-depleted aqueous plant extract.
 2. The method of claim 1, wherein the sorbent comprises one or more surface activated titanium oxide particles.
 3. The method of claim 1, wherein the sorbent comprises one or more chitosans.
 4. The method of claim 1, wherein the sorbent comprises one or more calcium phosphates.
 5. The method of claim 1, wherein the sorbent comprises one or more mercaptoalkyl-substituted silica gels.
 6. The method of claim 1, wherein the sorbent comprises one or more mercaptoalkyl-substituted mesoporous molecular sieves.
 7. The method of claim 1, wherein the sorbent comprises one or more finely ground γ-aluminas.
 8. The method of claim 1, wherein the sorbent comprises one or more photocatalytic titanium dioxide particles.
 9. The method of claim 1, wherein the sorbent comprises one or more Au-anatases.
 10. The method of claim 1, wherein the sorbent comprises ceria.
 11. The method of claim 4, wherein the calcium phosphate comprises one or more hydroxyapatites.
 12. The method of claim 5, wherein the mercaptoalkyl-substituted silica gel comprises mercaptopropyl-substituted silica gel.
 13. The method of claim 6, wherein the mercaptoalkyl-substituted mesoporous molecular sieve comprises a (3-mercaptopropyl)silane covalently bonded to a mesoporous silicate.
 14. The method of claim 13, wherein the mesoporous silicate comprises a uniform pore size of about 2 nm to about 50 nm and a mean surface area of about 500 m²/g.
 15. The method of claim 6, wherein the mercaptoalkyl-substituted mesoporous molecular sieve comprises a mercaptopropyl-substituted mesoporous molecular sieve having a mercaptopropyl loading of at least 4%.
 16. The method of claim 15, wherein the mercaptopropyl loading is at least 8%.
 17. The method of claim 16, wherein the mercaptopropyl loading is at least 16%.
 18. The method of claim 6, wherein the mercaptoalkyl-substituted mesoporous molecular sieve has an average pore size of about 5.5 nm.
 19. The method of claim 1, further comprising: contacting plant material with an aqueous extractant to form a first mixture comprising the aqueous plant extract and extracted plant material; separating the aqueous extract from the first mixture, leaving behind the extracted plant material.
 20. The method claim 19, further comprising: combining the heavy metal-depleted aqueous plant extract with extracted plant material to form a heavy metal-depleted plant material product.
 21. The method of claim 1, wherein the aqueous plant extract comprises aqueous tobacco plant extract.
 22. The method of claim 1, wherein the one or more heavy metals comprise one or more of cadmium, arsenic, lead, nickel, or selenium.
 23. The method of claim 20, wherein the plant material comprises uncured or cured tobacco.
 24. A heavy metal-depleted tobacco plant extract prepared by the process of claim
 21. 25. A heavy metal-depleted tobacco product comprising the heavy metal-depleted tobacco plant extract of claim
 24. 26. A smoking article comprising the heavy metal-depleted tobacco product of claim
 25. 27. The smoking article of claim 26, wherein the smoking article is a cigarette.
 28. The smoking article of claim 27, wherein the cigarette is a traditional cigarette.
 29. The smoking article of claim 27, wherein the cigarette is an electrically heated cigarette.
 30. A smokeless tobacco product comprising the heavy metal-depleted tobacco product of claim
 25. 